Many industrial products are produced by microorganisms grown in culture. Microorganism growth may be supported by soluble sugar molecules released by lignocellulosic biomasses. Lignocellulosic biomasses consist primarily of cellulose (polymers of glucose linked by β-1,4-glucosidic bonds), hemicellulose (polysaccharide composed of different five (C5)—carbon sugars and six (C6)—carbon sugars linked by variety of different β and α linkages) and lignin (complex polymer consisting of phenyl propane units linked by ether or carbon-carbon bonds). In some cases, lignocellulosic biomasses are subject to dilute acid hydrolysis during which hemicellulose is hydrolyzed to monomeric sugars (liquid stream) and the crystalline structure of cellulose is damaged, facilitating future enzymatic digestion (solid fiber). The liquid containing C5 and C6 sugars, so called hydrolysate, is separated from cellulose and lignin solids and can be fermented to various products such as ethanol. In addition to sugars however, hydrolysate also contains aliphatic acids, esters (acetate), phenolics (different compounds obtained from lignin hydrolysis) and products of sugar dehydration, including the furan aldehydes furfural and 5-hydroxymethyl furfural (5-HMF). Most of these compounds have a negative impact on microorganisms and can inhibit fermentation. Detoxification of the hydrolysate prior to fermentation is one measure that can be taken in order to avoid inhibition caused by toxic compounds present in the hydrolysate.
Various methods of detoxification have been tested, with alkaline overliming being efficient and cost effective. During the overliming process, the pH of the hydrolysate is temporarily raised, usually at an elevated temperature, from a pH of approximately 2 to a pH of between 9 and 10 through the addition of an appropriate amount of calcium hydroxide (lime). After some time, typically about 30 minutes, the pH of the hydrolysate solution is lowered through the addition of acid to a pH suitable for fermenting microorganisms. In the detoxification process, furan aldehydes are degraded and acids (mineral and organic) are neutralized.
Overliming has been known for a long time (Leonard and Hajny, 1945, Ind. Eng. Chem., 37 (4):390-395) and still is considered an efficient detoxification method. However, a significant drawback of the method is the considerable amount of loss of fermentable sugars that occurs during detoxification. See, e.g., Larsson et al., 1999, Appl. Biochem. Biotechnol. 77-79:91-103. The loss of fermentable sugars results in lower overall yields of fermentable products such as ethanol. In addition, the formation of insoluble calcium sulfate (gypsum) during detoxification is problematic. See, e.g., Martinez et al., 2001, Biotechnol. Prog. 17(2):287-293. Gypsum formation causes fouling and pipeline clogging, which significantly drive up maintenance costs. To overcome problems associated with calcium hydroxide, other bases have been attempted for the purpose of hydrolysate detoxification, which have met with varying levels of success. See, e.g., Alriksson et al., 2005, Appl. Biochem. Biotechnol. 121-124:911-922.
Accordingly, there is a need for new and improved processes to reduce fermentation inhibitors and detoxify hydrolysates obtained from lignocellulosic biomasses. In particular, there is a need for detoxification processes that are economically viable and provide detoxified hydrolysates capable of producing high yields of ethanol.